Download 60 Mendel, First Geneticist

Mendel, First Geneticist
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CHALLENGE
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n Activity 59, “Gene Combo,” you investigated a model for how inheritance works. This model is based on the work of Gregor Mendel. He
discovered the behavior of genes 40 years before scientists learned where
genes are located in the cell and almost 100 years before scientists discovered what the genes are made of!
Who was Gregor Mendel, and how did he discover the basis of heredity
by breeding pea plants?
Reading
Part A: Mendel’s Life
Gregor Mendel 1866
In 1865 an Austrian monk named Gregor Mendel published his work on
the behavior of genes. Mendel lived in a monastery that was devoted to
teaching and science, as well as religious matters. Mendel had prepared
to be a science teacher and had studied math, botany, and plant breeding
for several years. The monastery had an experimental garden for agricultural research. Research then, as now, included breeding varieties of plants
and animals in order to produce superior food and other products. Mendel became interested in the pea plants he was working on. He wondered
how the colors, shapes, and heights of offspring pea plants were related to
those of the parent plants. He began a careful study of how characteristics
are inherited in pea plants. Mendel’s discoveries depended upon research
about breeding plants, careful experiments, and creative thinking. He
understood that plant seeds are produced by sexual reproduction. He also
understood the mathematics of probability.
His experiments were planned so carefully and recorded so thoroughly that
anyone could repeat his procedure and confirm his findings. The patterns
he discovered apply to all sexually reproducing organisms, from petunias
to peas, from earthworms to humans. His discoveries have helped people
answer questions ranging from how to produce disease-resistant food crops
to explaining human diseases such as cystic fibrosis and sickle-cell disease.
Stopping to Think 1
What personal qualities do you think Mendel must have had that
helped him in his work?
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Activity 60 • Mendel, First Geneticist Part B: Mendel’s Experiments with Pea Plants
Plants in general, and pea plants in particular, were excellent organisms
for Mendel to study, for the following reasons:
• They grow into mature plants very quickly; in about 60–80 days
their seeds grow and develop into mature plants.
• They produce numerous seeds rapidly.
• They have many observable characteristics that come in just two
alternatives (such as purple or white flowers, yellow or green pods),
with no blending of these traits (no lavender flowers, no yellowish
green pods).
Mendel’s Results
Flower color
Seed color
Seed surface
Pod color
Original cross
(generation one)
purple x white
green x yellow
wrinkled x smooth
green x yellow
Generation two
offspring
all purple
all yellow
all smooth
all green
Generation three
offspring
705 : 224
(purple:white)
6,022 : 2,001
(yellow:green)
5,474 : 1,850
(smooth:wrinkled)
428 : 152
(green:yellow)
Mendel decided to breed extremely large numbers of pea plants and search
for simple patterns in the offspring. The table above shows the results he
obtained for four of the characteristics he studied.
Notice that Mendel did hundreds of crosses, and observed and counted the
offsprings’ traits for thousands of pea seeds, pods, and flowers. He then
analyzed all these results and applied his knowledge of statistics and probability to infer the behavior of individual genes.
Stopping to Think 2
a. What were the advantages for Mendel in using pea plants
for his breeding investigations?
b. Why did Mendel perform so many crosses for the same
­characteristics?
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Mendel, First Geneticist • Activity 60
As he looked at the data, Mendel noticed an interesting relationship. If he
calculated the ratio of the two traits in the third generation, he obtained a
ratio very near to 3:1.
yellow
yellow
For example, for seed color the ratio of yellow to green seeds can be
­calculated as
6,022 yellow       3.01 yellow
=
2,001 green           1 green
yellow
green
Seed Color Ratios
This is almost exactly a 3:1 ratio of yellow:green. This means that for every
1 green-seeded plant, there were almost exactly 3 yellow-seeded plants in
the third generation.
You can also express this in terms of the fractions 3⁄4 yellow and 1⁄4 green,
as shown in the pie graph at left.
Mendel concluded that not only did the green-seeded trait reappear in
the third generation, but the probability of a third-generation plant having that trait was 1⁄4—that is, about one green-seeded plant for every four
plants produced overall.
He found the same ratio with other characteristics also. The 3:1 ratio he
found for all of these different characteristics was the clue to how the parents’ genes combine in their offspring. Based on his analysis, he proposed
that:
• Each trait that appears in the second generation is the dominant
version of the characteristic. The trait that is “hidden” is called
recessive.
• Every plant has two copies of the gene for each characteristic. These
two copies are called alleles. (The evidence suggested there was more
than one copy of the gene for each characteristic in each plant, so
the simplest assumption that would work was to have two alleles,
or copies, per characteristic.)
• Each pea plant receives only one allele for each characteristic from
each parent and ends up with two alleles of its own.
He then used these ideas to argue that the 3:1 ratio is exactly what is
expected in the third generation for every plant characteristic. In the last
two activities, you have explored this proposal, using disks and coins as
models for genes.
Stopping to Think 3
Explain how the model in Activity 59, “Gene Combo,” works exactly
like Mendel’s explanation for his results with pea plants.
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Activity 60 • Mendel, First Geneticist Mendel published his results in a paper that was mostly ignored at the
time. Scientists and breeders failed to understand what he had done. Never
before had someone used mathematics to tackle a complex biological
problem. Mendel’s work was rediscovered and understood only after scientists realized that genes must be located on the chromosomes found in the
nucleus of the cell. You will learn about chromosomes in Activity 63,
“Show Me the Genes!”
Analysis
1. Based on Mendel’s results, which trait for each pea characteristic is dominant? In your science notebook make a table like the one below. In your
table, list the dominant and recessive traits for each characteristic.
Characteristic
Dominant Trait
Recessive Trait
Ratio Dominant:Recessive
Flower color
seed color
seed surface
pod color
2. a. Calculate to the hundredths place the ratio of dominant to recessive
for each characteristic in the third generation. Record the ratio for
each characteristic in the table you prepared for Question 1.
b. Why are the ratios not exactly 3:1?
3. Look at the pie chart on the previous page, which shows the ratio of
green-seeded and yellow-seeded offspring. Explain why a 1:3 ratio of
green-seeded plants to yellow-seeded plants is the same as a fraction of
1
⁄4 green-seeded plants.
4. Mendel performed his experiments on more characteristics than the
four shown in the circle graph, “Seed color ratios,” on the previous page.
Why was it important for him to look at more than one characteristic?
5. Reflection: People often think of mathematics as important to physics
and chemistry, but not to life science (biology). What is your opinion?
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